1. Field of Invention
The invention disclosed is an electrode comprising Ti-metal fiber wound on to a Ti-metal plate with an electrocatalytic coating that allows operation at a potential large enough to produce hydroxyl free radicals and oxidize substances dissolved in water or an electrolyte solution, and an electrochemical cell including such electrodes. An improved electrode coating sequence and coating procedure are also provided, providing increased service life and good current yield. Made of different materials, electrodes of this geometry may also be used in other process applications, in a fuel cell or as battery plaques.
2. Description of Prior Art
In U.S. Pat. No. 5,419,824 Weres and Hoffmann provided an electrode comprising a titanium metal substrate covered with a thin layer of titanium dioxide doped with about 4 mole percent of niobium in the +4 moxidation state. The single d-electron of the Nb+4 ions enters the conduction band of the mixed metal oxide, making the mixed oxide an heavily n-doped semiconductor. In U.S. Pat. No. 5,364,508 Weres and Hoffmann disclosed use of this electrode as an anode to generate hydroxyl free radical by oxidizing water and to oxidize organic substances dissolved in water. In U.S. Pat. No. 5,439,577 Weres and Hoffmann provided a water purification device utilizing the electrodes provided in U.S. Pat. No. 5,419,824 and an electrolytic cell wherein these electrodes are made by applying the doped titanium dioxide layer to titanium sheet, and assembled in a bipolar array.
A detailed electrode coating procedure was provided in U.S. Pat. No. 5,419,824. A xe2x80x9cwhite slurryxe2x80x9d coating composition was prepared, comprising hydrous titanium dioxide (the precursor of anatase pigment which has been precipitated from titanium sulfate solution and washed, but not dried or calcined) dispersed in water. The water soluble compounds diammonium bilactatotitanium (commercially available) and ammonium niobate were added in the correct proportions to cement the slurry and provide the desired level of Nb-doping. An xe2x80x9covercoatxe2x80x9d solution was also used, comprising an aqueous solution of the same titanium and niobium compounds. The Ti-metal substrate was dipped into the xe2x80x9cwhite slurryxe2x80x9d composition, then baked in air at 400xc2x0 C. to dry and bake on the slurry. About three coats of the white slurry were applied in this way, followed by three layers of xe2x80x9covercoat,xe2x80x9d which cemented the slurry coat. Finally, the electrodes were annealed at 650-800xc2x0 C. under hydrogen to reduce the niobium in the coating to the +4 oxidation state, conferring the desired semiconductive properties upon the electrode coating. Adding a bit of water vapor to the hydrogen inhibits hydrogen absorption into the Ti-metal substrate, and small electrodes in the form of disks or rods may be produced in this way. However, annealing plate electrodes under hydrogen warps them severely, and fiber electrodes are embrittled and practically destroyed. Therefore, the utility of the electrode coating method revealed in U.S. Pat. No. 5,419,824 is limited to producing small laboratory test electrodes. Also, electrodes coated in this manner fail after a few days of continuous operation due to passivation of the Ti-metal surface beneath the semiconductive oxide coat, making them useless for practical application. Even with periodic reversals of current, an electrode made of Ti-fiber cannot be operated in bipolar mode, because take-up of hydrogen while cathodically polarized embrittles and eventually destroys the fiber.
In U.S. Pat. No. 3,878,083 De Nora et al. provided a titanium electrode coated with a mixture of iridium dioxide and tantalum pentoxide. In U.S. Pat. No. 4,839,007 Kxc3x6tz et al. provided a method of purifying industrial waste water using an anode comprising a Ti-metal substrate coated with tin dioxide doped (in the preferred embodiment) with antimony. This coating composition allows the electrode to operate at potential high enough to oxidize organic materials dissolved in water. In U.S. Pat. No. 5,364,509 Dietrich described a titanium anode with a two layer coating. The first coat comprises a mixture of IrO2 and Ta2O5, and the second coat comprises SnO2 doped with Sb.
In U.S. Pat. Nos. 4,444,642 and 4,528,084 Hinden and Beer teach using a solution of iridium trichloride and HCl in an alcohol solvent to apply a protective precoat, noting that the solution should attack the Ti-metal substrate, producing a thick oxide layer comprising IrO2 and TiO2, intimately mixed. This coating solution is strongly reducing and depassivates the Ti-metal surface, causing it to corrode. In trying to use this solution, we also noted that it spoils rapidly once used, probably because Ti+3 produced by corrosion of the Ti-metal reduced the iridium in solution, causing it to precipitate. U.S. Pat. No. 3,878,083 teaches application of a coating comprising IrO2 and Ta2O5 using a solution of IrCl3 and TaCl5, in hydrochloric acid. This coating solution is very weakly oxidizing. Scanning electron microscopy of Ti-fiber electrodes that we precoated using a solution comprising H2IrCl6 and TaCl5 in hydrochloric acid (which is more strongly oxidizing and thereby less corrosive against Ti than the solution recommended in U.S. Pat. No. 3,878,083) revealed that some fibers had thick coatings on them, indicating depassivation and corrosion of the Ti-metal substrate, while other fibers had very thin coats. Because the diameter of the fibers is small, corrosion, if it occurs, can dissolve a large fraction of the fiber""s mass, and the thick mixed oxide coating produced fills in the grooves typically present in the surface of the fibers, decreasing their effective surface area.
In process electrochemistry, increasing electrode surface area improves the kinetics of the electrochemical process at low reactant concentration. Increased surface area also decreases the true current density at the surface in proportion, allowing the cell to operate at lower voltage and increasing the service life of the electrode. In batteries, increased surface area of the electrode plaques provides improved contact with the active material, improving energy storage efficiency. In practice, large surface area process electrodes and battery plaques are very similar and their design is governed by much the same criteria, allowing technology to be usefully and easily transferred between the two fields.
In U.S. Pat. No. 3,895,960 Brown et al. provided an electrode plaque made by compressing and diffusion bonding iron fibers, attaching a current collector by mechanical means or by welding, and plating the entire assembly with nickel to provide the needed electrocatalytic surface properties. In Brown""s Example 1, iron fibers with length:diameter ratio of about 1,900 were used to produce an electrode plaque with 95% porosity, 0.025 inch thickness, and specific area 100 cm2/cm3. In U.S. Pat. No. 3,835,514 Pollock provided a similar electrode plaque with L:D of 800 to 8000: 1, porosity of 70 to 97% and a diffusion bonded bus connector.
In U.S. Pat. No. 4,331,523 Kawasaki described electrodes suitable for water electrolysis comprising a perforate current collector, preferably titanium expanded mesh or titanium perforated plate coated with platinum group metals, with a xe2x80x9cfibrous assemblyxe2x80x9d pressed against it to provide large surface area. He noted that the fibrous assembly could comprise a diffusion bonded xe2x80x9cwebxe2x80x9d of titanium fibers coated with platinum groups metals. (Here and throughout, we use the term xe2x80x9cplatinum group metalsxe2x80x9d to mean the metallic elements Ru, Rh, Pd, Os, Ir and Pt and also their oxides.) Kawasaki did not specify L:D, porosity or specific area of the xe2x80x9cfibrous assemblyxe2x80x9d in his electrodes, but his examples suggest values similar to those taught in U.S. Pat. Nos. 3,895,960 and 5,294,319.
In U.S. Pat. No. 4,708,888 Mitchell et al. described an electrode produced by applying an electrocatalytic coating to a fine titanium expanded mesh, then spot welding or metallurgically bonding current distributor members (also coated Ti) to the coated mesh.
In U.S. Pat. No. 5,294,319 Kaczur et al. combined and improved upon the elements of U.S. Pat. No. 3,895,960 and 4,331,523 to provide an electrode comprising a mat of titanium fibers of at least two distinct diameters with length:diameter greater than 1000:1 coated with platinum group metals and spot welded to a similarly coated titanium plate.
Metallurgically bonded fibrous electrode structures as provided in U.S. Pat. Nos. 3,895,960 and 4,331,523 are poorly suited to our application because a slurry coating composition would not penetrate into the structure of the electrode plaque and coat the fibers uniformly. The same is true of the electrode provided in U.S. Pat. No. 5,294,319 comprising fibers spot welded to a plate. The same problem would preclude reprocessing and recoating spent electrodes. Also, production costs would be high, and the electrodes would be highly susceptible to fouling by particulates in a waste water treatment application.
The electrode provided by Coin et al. in U.S. Pat. No. 5,783,050, comprising multiple layers of Ti-expanded mesh wound on a Ti-plate with an electrocatalytic coating applied to the assembly, appears to solve this problem. However, the surface area of the expanded mesh is not very large, and applying many layers of expanded mesh to provide a large surface area would make the electrode quite thick. The current needs to flow a considerable distance through electrolyte to reach all active surfaces throughout the thick mesh portion of the electrode. Because electrolyte conductivity in a typical waste water treatment application is small, an electrode with this geometry would operate with uneven current distribution, where most of the current is bunched at the outermost layer of the electrode.
The electrode provided by Morin in U.S. Pat. No. 4,680,100 comprises a thick tow of thousands of very fine nonmetallic fibers coated with a thin layer of metal and wound on a nonconductive plastic support member with electrical connectors attached to the ends of the tow using solder. This electrode cannot be made of titanium or another valve metal, because titanium cannot be plated on to a nonmetallic fiber substrate, and titanium fiber tow is not available with the very small diameter and very large fiber count disclosed. If Ti-fiber tow of this geometry were available, coating it by dipping and baking would both embrittle and cement the very fine fibers; in particular, attempting to apply a slurry coating would cover the tow with a crust leaving most of the fibers inside the tow uncoated and practically inaccessible to electrolyte. If a Ti-fiber electrode with semiconductive oxide coating could be made with this geometry, it would be practically inoperable. With the modest electrolyte conductivity typical of most waste water treatment applications, current would not be able to penetrate into the thick tow much below its exposed surface, and most of the fibers in the tow would remain inactive. Also, the resistance of the very fine titanium fibers would be so large that the current would not travel more than a few inches along the length of the tow, causing most of the length of the tow to remain inactive. Conduction perpendicular to the fibers would be small, as the semiconductive coatings relevant to our application exhibit a contact resistance, effectively blocking current from penetrating more than a few fibers in the perpendicular direction. It would be difficult to solder electrical connectors to the ends of the tow, because solder doesn""t wet the slurry coat. If a solder bond were achieved, the solder would be wetted by electrolyte and would corrode, contaminating the water being treated with tin and lead.
While we believe the theoretical explanations set forth herein to be true, we do not wish to be bound by them.
Herein we improve upon the electrodes described in U.S. Pat. No. 5,419,824. A reformulated outermost oxide coating is provided (hereinafter called the xe2x80x9cslurry coatxe2x80x9d) comprising fine particles of TiO2 doped with Nb in the +4 oxidation state cemented with an infilling matrix of Sb-doped TiO2. Additional coating layers applied to the Ti-metal substrate before applying the slurry coat provide a long service life, and favor good current efficiency when the electrode is operated as an anode.
First, a xe2x80x9cprecoatxe2x80x9d comprising IrO2 and Ta2O5 is applied. The precoat is very stable and electrocatalytic for generation of oxygen at moderate anodic potential. It protects the anode from failure by passivation of the Ti-metal substrate beneath the slurry coat. Any current that reaches the precoat through cracks in the slurry coat is discharged at a relatively low potential by generating oxygen, and the potential of the Ti-metal surface never rises high enough to create an anodic oxide layer thick enough to impede operation of the anode.
A xe2x80x9csealing coatxe2x80x9d comprising SnO2 doped with Sb is applied over the precoat. The sealing coat adheres well to the precoat, and the slurry coat adheres well to the sealing coat. The result is a well adherent slurry coat with few cracks that extend through to the precoat. Minimizing the amount of current that leaks from the electrolyte solution directly to the precoat through cracks in the slurry coat minimizes the amount of current that is wasted by generation of oxygen. The sealing coat is itself capable of operation at an anodic potential large enough to oxidize some substances. Thus, leakage of current from the electrolyte through the slurry coat to reach the sealing coat does not depolarize the anode to nearly the same degree as would leakage of current to reach the precoat. Thereby, a large fraction of the anode current actually passes through the outer surface of the slurry coat creating hydroxyl free radical, and the current yield of the anode is good. (Current yield is proportional to Chemical Oxygen Demand removed from the electrolyte, divided by the electric charge passed through the cell. At 100% current yield, passing 1 Coulomb through the cell would remove 0.083 mg of COD.)
Adequate service life and current yield require that most of the surface of the Ti-metal substrate be covered with an moderately thick semiconductive oxide coat, and xe2x80x9cslurry coatingxe2x80x9d is the only practical way to accomplish this. Also, the xe2x80x9cslurry coatxe2x80x9d is believed to be ceramic-like in its microstructure rather than glass-like, whereby cracks tend not to penetrate the entire thickness of the slurry coat.
Herein we provide an electrode comprising Ti-fiber tow (preferably about 200 fibers, approximately 25 micrometers in diameter) wound around a Ti-metal plate. The electrode is easily manufactured by directly winding the tow on to the plate using a suitable winding machine. The Ti-metal fiber provides large active surface area, but the diameter of the fiber is large enough to allow coating without embrittlement or excessive cementation. Cementation that occurs is easily countered by simple mechanical means; for example, by sonication between layers of precoat and sealing coat, or by lightly rubbing and rolling the tow against the plate to separate the fibers between layers of slurry and overcoat. When the coating fails, poorly adherent material can be removed by sonication and the plates can be recoated. Because the fibers are not in any way fused together, good permeability is retained.
Anodes of this description are most conveniently stacked in alternation with flat plate cathodes, separating anodes and cathodes using a plastic coated fiberglass screen to prevent short circuiting. Compressing the stack presses the fibers against the Ti-plate providing good electrical contact along the entire length of the tow, and producing a thin reaction zone which favors even distribution of current over the surface of all fibers. Forcing flow perpendicular to the length of the tow provides good contact of the electrolyte with the coated fiber, and purges gas bubbles from between the fibers. This electrode and electrolytic cell are well suited for the purification of waste waters by oxidation of organic compounds dissolved in the water.
Alternatively, electrical contact can be provided by securing the fibers to the edges of the plate by slipping a small extruded plastic channel over the edge of the electrode. This arrangement is useful when operating in more concentrated electrolyte with a physical gap present between adjacent electrodes.